"The death from three US drone attacks in North Waziristan has gone up to at least 15," a senior security official in the north-western city of Peshawar said, revising the earlier toll of eight.

Another security official confirmed the new toll saying "now we have reports that the attacks left 15 dead and several injured."

The official feared the death toll may rise.

"We are collecting details and the toll may rise further," he added.

The strikes in Tundar village in North Waziristan, known as a bastion of Taliban and al Qaeda, came a day after Pakistan summoned a US diplomat to protest over the attacks, calling them "unlawful".

"US drones fired six missiles, two each on the three militant compounds," the official said.

He said several drones were flying over the village at the time of the strike.

A third security official confirmed the strikes and casualties, adding that the identities of the dead were not immediately clear, also confirming that the death toll may rise.

"We are gradually getting information from the area of the new strikes, which is mountainous and covered by thick forest," he said.

Officials said the area targeted is used by militants belonging to Tehreek-e-Taliban Pakistan (TTP), Afghan Taliban allied with Haqqani network and Hafiz Gul Bahadur group.

Pakistan has repeatedly criticised American drone strikes in its territory, calling them counter-productive.

Attacks by unmanned US aircraft are deeply unpopular in Pakistan, which says they violate its sovereignty and fan anti-US sentiment, but American officials are said to believe the attacks are too important to give up.